The present invention relates to a method for measuring the insulation resistance in power supply lines and apparatus therefor.
Present practice for measuring the ground insulation of a low voltage power line (hereinafter simply referred to as "line") for terminal consumers is that the line is interrupted from the main supply of voltage and the ground insulation resistance (hereinafter referred to as simply insulation or insulation resistance) is determined by an insulation measuring meter such as a Megger (Tradename) and by using this measured value the insulation condition is judged. However, a method and device able to measure the insulation to the same extent of accuracy while the line is in use (hereinafter referred to as an active line) have been requested.
The ground leakage current in a line, in which one end of the secondary circuit of the transformer is earthed, returns to the ground wire and among such a leakage current, a charging current for the stray capacitance, between ground hereinafter referred to as stray capacitance, is included. This occurs even in a line having good insulation resistance. Accordingly, if the insulation is to be measured by utilizing the leakage current, it is necessary to detect only the leakage current caused from a bad insulation and by removing the component caused from the stray capacitance. Furthermore in a measurement using the leakage current returning into the ground wire, there are the following problems. Namely in a single phase 3-wire circuit, there is sometimes caused simultaneous bad insulation for the two non-grounded wires among the 3-wire circuit. In this case the leakage currents mutually compensate for each other in the ground wire so that a correct detection of bad insulation is not possible only from the measurement of the leakage current.
As known measuring methods for the insulation resistance, the followings are considered.
(a) A method for measuring the insulation resistance by measuring the dc current returning into the ground wire by inserting an auxiliary dc voltage source in series with the ground wire.
(b) A method for measuring the insulation resistance by detecting a returning current of a low frequency ac measuring current by inserting a low frequency ac voltage source in series with the ground wire and deriving the effective component caused by the insulation resistance in the current.
(c) A method for detecting the effective component in the current returning through the ground wire.
In the methods of (a) and (b), it is required to insert the measuring voltage source in series with the ground wire so that this might cause a possible high ground resistance. Also in a case of a ground short circuit accident, there is a danger of breakage or destruction of the inserted voltage source. These methods are troublesome in requiring much work in order to insert the measuring voltage source in the ground wire. The method (c) can not be used for a single phase 3-wire line by the reasons set forth above.